Many industrial processes require the mixing of different fluids or the dilution of one fluid with another. For example, liquid polyelectrolytes used in various water treatment processes must sometimes be diluted with water in a volumetric proportion typically of 200 to 1 for commercial applications, but this can vary. Due to the large amount of water diluent required, it is often significantly less expensive to transport only the polyelectrolyte and to mix the fluids on site, so that the transportation cost is substantially reduced. Many liquid polyelectrolytes are generally not easy to mix with water of their high viscosity and/or chemistry which inhibits mixing. In other instances, processes require mixing a single fluid such that it is homogeneous before the fluid can be used.
Generally, when two different fluids are mixed, such as a liquid polyelectrolyte and water, each fluid is initially in a region composed purely of itself, surrounded by another region composed purely of the other fluid. In order to mix the fluids, the regions are brought together. A mixing surface area exists between the two pure regions. Mixing results as molecules from one pure region transfuse into the other pure region This can happen only at the mixing surface. Consequently, increasing the mixing surface area per unit volume accelerates mixing for a particular volume of a fluid in a diluent. Generally, the total surface area per unit volume is increased as a single volume of one fluid is divided into more smaller volumes.
An increase in mixing surface area can be achieved by introducing a shear force to the fluid in a diluent. This shear force moves part of the fluid at a different velocity than other parts of the fluid, breaking up the single pure region into more, volumetrically smaller regions. As a result, the mixing surface area per unit volume for the particular fluid volume is increased.
Shear can be introduced to a fluid in several ways. One way to introduce shear is to draw a member through the fluids, mechanically breaking up the pure region. This is similar to stirring oil and vinegar with a spoon. Another way to introduce shear into a fluid is by creating turbulence in the fluid. The turbulence creates fluid streams of different speeds and directions, operating to move parts of the pure region in different directions simultaneously, thereby creating more smaller pure regions and, thus increasing the mixing surface area per unit volume of fluid. When a fluid has a high viscosity, such as with a liquid polyelectrolyte, it is more reluctant to be broken up into smaller regions. Consequently, mixing is more difficult.
In certain applications requiring viscous polyelectrolyte fluids to be diluted in water, the mixed solution must be substantially homogeneous. Further, the mixing should be done in a short time so that the mixture can be used at once without requiring significant storage space to allow time for the mixture to "age."
One apparatus for mixing liquid polyelectrolytes and water is shown in U.S. Pat. No. 4,886,368 to L. Tony King, which describes a device that "smears" the two fluids, proposing to increase the mixing surface area per unit volume by introducing the liquid polyelectrolyte to the water as a thin film, without much thickness. In the '368 patent, a drive shaft rotates within a cylindrical chamber. Grooves on the outer circumference of the drive shaft run the length of the chamber. The space between the outer diameter of the drive shaft and the wall of the chamber is small, i.e., on the order of 0.005 inches. Water is introduced into the chamber, flowing over the drive shaft and through the grooves and out of an outlet hole at the rear of the chamber. A liquid polyelectrolyte is introduced radially into the chamber at a point intermediate the chamber and the drive shaft. Because the annular gap region between the drive shaft and the wall of the chamber is so small, the '368 patent states that water and the polyelectrolyte are "smeared" together, i.e., a thin layer of polyelectrolyte and a thin layer of water are pressed together, inducing mixing.